If the cavity extends aft and is lower then the rest of the Dorade box I would close it off to keep water out of there. I'd use epoxy and cloth and maybe even some gel-coat to cover it.
As to attaching a picture when replying you need to click on the 'Go Advanced' button below and click on 'manage attachments' or something like that.

"The cure for anything is salt water~ sweat, tears, or the sea." ~Isak Denesen

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